The present invention relates to a scanning near-field optical microscope having an optical resolution less than the wavelength and, more particularly, to a scanning near-field optical microscope for observing locally excited emission of light from a semiconductor device or the like under a low-temperature condition.
A so-called scanning near-field optical microscope has been known as an optical microscope having an optical resolution less than the wavelength. Examples of this kind of microscope are disclosed in Patent Unexamined Publication No. 291310/1992, entitled, "Near-Field Scanning Optical Microscope and its Applications", by Robert Erik Betzig and in Patent Unexamined Publication No. 50750/1994, entitled, "Scanning Microscope Including Force-detecting Means", by Robert Erik Betzig. An example of a low-temperature near-field optical microscope capable of cooling a sample is disclosed in Rev. Sci. Instrum. 65(3), 1994, pp. 626-631, by Robert D. Grober et al. An example of a near-field optical microscope using a quartz oscillator is disclosed in Appl. Phys. Lett. 66(14), 1995, pp. 1842-1844, by Kaled Karai et al. These instruments are outlined below.
The near-field scanning optical microscope is also known as NSOM. FIG. 2 is a schematic view of the prior art near-field scanning optical. microscope. The tip of an optical fiber 310 is machined into a tapering form,70. An aperture less than the wavelength is formed at the tapering tip of the probe. A sample stage 20 is placed on an XYZ stage 50. A sample 30 is set on the sample stage. It is held close to the sample surface, using an XYZ fine motion device 40, and a certain region is raster-scanned. The optical fiber probe 70 is moved parallel to the sample surface, using a fine motion device 40. A horizontal force from the sample surface, or a shear force, acts on the tip of the probe. Thus, the state of the vibration of the probe varies. To measure the state of vibration of the probe 70, laser light (not shown) used for position control is directed at the tip of the probe, and the shadow of the probe 70 is directed through a pinhole 120 by a lens go and detected by a photomultiplier 110 of a position sensitive detector 80. The distance between the sample surface and the tip of the probe is controlled, using the fine motion device 40, so that the shear force is kept constant, i.e., the rate at which the amplitude or phase varies is kept constant. The shear force drops rapidly with the distance from the sample. Utilizing this, the distance between the sample surface and the tip of the probe is kept constant on the order of nanometers. Under this condition, laser light from a laser light source 60 is introduced into the fiber 310, using a lens 150, to illuminate the sample surface from the aperture at the tip. A part of light reflected or transmitted is detected by conventional optics (not shown). As described thus far, the resolution of NSOM depends on the size of the aperture at the tip of the probe. Since it is easy to form apertures less than the wavelength (e.g., less than 100 nm), high resolution less than the wavelength can be realized.
FIG. 3 is a schematic view of the prior art near-field optical microscope. C is a sample and a piezoelectric scanner for scanning the sample. D is an optical fiber probe and a piezoelectric device for vibrations. The optical fiber probe is machined into a tapering form and formed with an aperture less than the wavelength at its tip. The optical fiber probe is vibrated parallel to the sample surface, using the piezoelectric device for vibrations. A horizontal force from the sample surface, or a shear force, acts on the tip of the probe. Thus, the state of the vibration of the probe varies. To measure the state of vibration of the probe, laser light (not shown) is used. A is a diode laser. B is a lens. F is a photodiode detector. Laser light for position control is directed at the tip of the optical fiber probe. The shadow of the probe is detected by the lens and the detector.
The distance between the sample surface and the tip of the probe is controlled, using the piezoelectric scanner C, so that the shear force is kept constant, i.e., the rate at which the amplitude or phase varies is kept constant. The shear force drops rapidly with the distance from the sample. Utilizing this, the distance between the sample surface and the tip of the probe is kept constant on the order of nanometers. Under this condition, laser light (not shown) used for near-field optical measurement is introduced into the fiber D, to illuminate the sample surface from the aperture at the tip. A part of reflected light is detected by conventional optics (not shown). The resolution depends on the size of the aperture at the tip of the probe. Since it is easy to form apertures less than the wavelength (e.g., less than 100 nm), high resolution less than the wavelength can be realized. A cryostat is used to cool the sample. E is a chamber and an optical window in the cryostat. By placing the sample inside the cryostat, the sample can be cooled down to liquid helium temperature. This structure permits near-field optical measurement while the sample is cooled.
FIG. 4 is a schematic view of the main portion of the prior art "near-field optical microscope using a quartz oscillator." Indicated by 400 is an optical fiber probe. Indicated by 410 is a quartz oscillator. The optical fiber probe is adhesively bonded to the quartz oscillator, which is made to resonate by a piezoelectric device (not shown) for vibrations. Vibration of the quartz oscillator vibrates the optical fiber probe. As the tip of the probe comes close to the sample, a horizontal force from the sample surface, or a shear force, acts on the tip of the probe, thus varying the state of vibration of the quartz oscillator. The state of vibration of the quartz oscillator is measured by measuring electric charge generated by the piezoelectric effect of the quartz. The distance between the sample surface and the tip of the probe is controlled, using a piezoelectric scanner (not shown), so that the shear force is kept constant, i.e., the rate at which the amplitude or phase varies is kept constant. The shear force drops rapidly with the distance from the sample. Utilizing this, the distance between the sample surface and the tip of the probe can be kept constant on the order of nanometers.
The prior art scanning near-field microscope described above has the following disadvantages. In near-field scanning optical microscopy (NSOM), laser light is directed at the sample surface near the tip of the optical probe, and an image (shadow) of the tip of the probe is detected from the reflected light to detect the shear force. Therefore, the amount of reflected light is readily affected by the topography of the sample surface and by the reflectivity. Hence, it is difficult to measure the amplitude of vibration, and it is difficult to precisely measure the surface contour. Furthermore, it is not easy to align the laser light and so the data reproducibility has posed problems. Furthermore, the measured region on the sample surface is illuminated with the laser light used for detection of the shear force, as well as with exciting light from the optical probe. This increases the background noise. Additionally, it is difficult to remove the noise. In spectroscopic measurement, it is impossible to measure wavelengths close to the wavelength of the laser light used for the shear force. Moreover, an optical fiber or the like is necessary to remove the laser light used for the shear force. This leads to a decrease in the amount of emitted light contributing to detection. As a result, the SIN of the data deteriorates.
The low-temperature scanning near-field optical microscope uses laser for detection of a shear force in the same way as the above-described near-field scanning optical microscope (NSOM). This makes it difficult to measure the amplitude of vibration. In addition, it is difficult to measure the surface topography precisely. Furthermore, it is not easy to align the laser light. Data is reproduced with insufficient reproducibility. Furthermore, the measured region on the sample surface is illuminated with the laser light used for the shear force, as well as with exciting light from the optical probe. This increases the background noise. In spectroscopic measurement, it is impossible to measure wavelengths close to the wavelength of the laser light used for the shear force. Moreover, an optical fiber or the like is necessary to remove the laser light used for the shear force. This leads to a decrease in the amount of emitted light contributing to detection. As a result, the SIN of the data deteriorates. The sample is positioned inside the cryostat, while the optics including the laser light for detection of a shear force is placed outside the cryostat. Therefore, the optical window tends to attenuate the amount of laser light. In consequence, the measurement is rendered difficult. Additionally, flow of low-temperature helium gas or liquid helium tends to cause the laser light to fluctuate. Consequently, it is difficult to control the position of the optical probe. Further, it is difficult to remove the aberration, because a light-gathering system using reflective mirrors is used. As a consequence, blurring of the image has presented problems.
In the near-field optical microscope using a quartz oscillator, the portion where the quartz oscillator and the optical fiber are adhesively bonded together tends to be a microscopic region (e.g., a square region about 100 m square). It is difficult to perform the bonding operation. Furthermore, the characteristics of the quartz oscillator device are easily affected by the amount of adhesive, the hardness, the location at which they are bonded, and other factors. Thus, it is difficult to obtain an oscillator sensor with high reproducibility. For these reasons, it has been difficult to use the instrument in industrial applications. Where the optical probe is replaced, the quartz oscillator must also be replaced. This gives rise to an increase in the cost. In addition, near-field optical measurement with high reproducibility has been impossible to perform.